High voltage cutoff with No pic

[Gundy]

High voltage cutoff with No pic
I have been charging my A123 on a nicad charger, just disconnecting the battery when it gets to 7.4 (2 cell)
But I have to stand there and watch the meter… that sux.
So, I need a circuit that will react when it gets to the voltage I want.
3.7, 7.4, 11.1, 14.8
This way I could use a relay to open the battery connection to the charger.
Using a simple circuit to check the voltage at the battery should be simple. This way I don’t need to limit the current or the voltage, the charger does that.
I just want something to check the voltage and when it gets to 7.4 (or what ever) it trips a relay.

any circuit ideas??

[Dan Baldwin]

This cuttoff is adjustable from about 6 volts to about 18 volts. It will not work for a single A123 cell. If this is what you are looking for, I'll breadboard it tomorrow with the values shown to make sure it does what I think it will.

Dan

[AndyOne]

What's an A123, it's not a lithium polymer battery is it?

If it is, this isn't any way to charge it, first of all the normal charge voltage for a 2 cell battery of this type is 8.4 volts and it isn't terminated by voltage. In order to get a full charge in it needs to be held with a constant voltage limit whilst the current falls to about c/10.

Get a proper charger there really cheap these days and much safer than relying on your attention for end of charge switch off.

Andy.

[Gundy]

The new miracle battery :rolleyes:
It’s the new battery that Dewalt is using in its new 36V tools.
Each battery is 3.3V 2300mah and puts out like 40 amps. It can also be charged at 10amps and will not explode.
I am no expert on them, but I bought 2 Dewalt packs (20 cells) and made some 4cell packs (13.2v)
I am using them in my 48” jet and 48” slow-stang.
Slow-Stang maiden (http://www.rcgroups.com/forums/showthread.php?t=625455)
48" Blu-Core F-15 Maiden video (http://www.rcgroups.com/forums/showthread.php?t=603245)

They charge in about 10 min on my old ni-cad charger. But I have to watch them.
If I had a meter with an alarm on it that would work to.

Later,
Gundy

[Gundy]

This cuttoff is adjustable from about 6 volts to about 18 volts. It will not work for a single A123 cell. If this is what you are looking for, I'll breadboard it tomorrow with the values shown to make sure it does what I think it will. Dan
I was thinking about a zener circuit, but I have not played with any. Plus living in the back woods I need to order all my parts in. so I needed a place to start.
This looks good but you’re going to have to explain the shunt regulator to me.

[Dan Baldwin]

The circuit is not a shunt regulator, it is an adjustable high voltage cut off using a TL431 adjustable zener diode.

If you look at the circuit without the TL431 connection to the junction of R5 and R6, you will see that Q1 is biased on by the voltage divider formed by R4, R5, and R6. When Q1 is turned on, the 2N3906 (sorry, forgot to add the Q2 label) is biased on as well by the voltage divider formed by R7 and R8. The 2N3906 is being used in voltage follower mode, and the 5.1 volt zener diode limits the voltage applied to the relay to about 4.5 volts regardless of the voltage of the pack being charged. Resistors R1, R2, and R3 form a voltage divider for the voltage sense pin of the TL431. If the voltage at the sense pin of the TL431 goes above 2.5 volts, the TL431 will turn on, bringing the voltage at the junction of R5 and R6 to about 2 volts, which turns Q1 off, which also turns the 2N3906 off, which also turns the relay off, dropping out the charger. Resistor R10 adds about half a volt of hysteresis so that when the relay drops out and the voltage on the pack falls, the circuit does not turn on again.

Should I breadboard it?

Dan

[Gundy]

YES YES YES!

It so funny, when you explain the circuit I can follow along just fine... but to design it, that is a whole different story.

[Dan Baldwin]

Stay tuned. There will be some changes.

Dan

[Dan Baldwin]

I edited the new schematic into my previous post. I changed the design quite a bit. The cut off voltage is adjustable from about 6.2 volts to about 20 volts. Q2 is operated in linear mode, and it has the full relay current flowing through it, so it got quite hot at higher voltages using a 2N3906, so a TO220 would be a better choice. The only non-darlington PNP TO220 I had on hand was a TIP42, so that's what I used. Even using the TO220 transistor it may be necessary to use a small heat sink, depending on how much current your relay draws. My 5 volt relay draws about 50 MA which is about .75 watts through Q2 at 20 volts. The hysteresis is about .7 volts at 6 volts, and about 1.25 volts at 20 volts. Hysteresis can be increased by using a smaller resistor for R8, or decreased by using a larger value.

Let me know if you need an explanation of the new design, since it works a bit differently.


Dan

[Gundy]

yes you better walk me through it just to make sure I got it.
I am going to try it this weekend.

Later,
Gundy

[Dan Baldwin]

I'll start at the other end this time.

The 5.1 volt zener, R6, and Q2 are a primitive voltage regulator that keep a constant 4.5 volts across the relay. When Q1 turns on, it reduces the voltage across the zener to near zero, effectively turning off the regulator circuit and dropping out the relay. As before, when the voltage on the sense lead of the TL431 goes over 2.5 volts, the cathode connection(lead connected to R5) drops to about 2 volts, turning Q1 on.

I have a couple of suggestions for part substitution. If a 5.6 volt or 6 volt zener were used in place of the 5.1 volt zener, the relay would have closer to 5 volts across it.
The TIP42 transistor worked fine on my breadboard, but it may be possible to get one that won't work in this circuit because it doesn't have enough gain. I suggest using a D45H8 or D45H11 instead if you can find them.

Dan